1. Field of the Invention
The present invention relates to a battery charging control method, and in particular, to a technique effective for reducing the time necessary for charging a low-temperature battery.
2. Description of the Related Art
Conventionally, hybrid vehicles employing a motor in addition to the engine as a driving source, or electric vehicles employing only a motor as a driving source, are known.
The motors of such vehicles are driven by electric power supplied from a battery; thus, when the remaining charge of the battery decreases, the battery must be charged.
In order to charge a battery which has discharged electricity, it is recommended to charge the battery at one tenth of its rated amount (i.e., 0.1 C.) for approximately 10 hours, and rapid charging of batteries is rarely performed because such rapid charging may cause degradation of the battery and reduction of the battery life.
In particular, the batteries of electric vehicles are generally charged by electric power supplied at night, and the charged power is used in the next day.
However, rapid charging of the battery is still required when a driver has an urgent need to use the vehicle at night, or when a driver wants to extend the driving distance by recharging the battery.
For rapid battery charging, if the battery temperature is below an appropriate range of temperatures for battery charging, that is, lower than Tmin in FIG. 10, the conductivity of the electrolyte decreases and the resistance of the electrolyte rapidly increases, thereby increasing the voltage for charging (see the voltage V in FIG. 10).
In particular, the resistance of lithium organic-solvent electrolytes at room temperature is higher than that of aqueous-solution-type electrolytes; thus, the rate of increase of the resistance of lithium organic-solvent electrolytes is very high at low temperatures.
In order to prevent the electrolyte from decomposing due to an increase of the charging voltage (in the case of a lithium battery), or to prevent the generation of gaseous oxygen (in the case of an alkali battery), an upper limit of the charging voltage is determined, and if the charging voltage exceeds the upper limit (see Vmax in FIG. 10), the charging current (see current I in FIG. 10) is decreased so as to perform constant-power charging. Accordingly, the charging operation requires a long time, or the amount of charging may not be sufficient.
In order to solve these problems, a charging control method, in which the charging operation is started after the battery is heated using a heater or the like, may be employed. However, in this method, a dedicated heating system is necessary.
Therefore, when a battery is built into a hybrid vehicle or an electric vehicle, a space for providing a heating system must be secured in the layout of the vehicle, and additionally, the weight of the vehicle increases. As a result of these drawbacks, this method is not preferable.
In consideration of the above circumstances, an object of the present invention is to provide a battery charging control method for reducing the time necessary for charging a battery at a low temperature without providing an additional heating system.
Therefore, the present invention provides a battery charging control method wherein when the temperature of a battery is lower than a predetermined temperature (e.g., 10xc2x0 C. in the embodiment explained below), the battery is first heated by a pulsed charging and discharging operation comprising alternately executing charging and discharging operations, before the battery is charged.
According to this method, the battery is heated due to a heating effect based on Joule heat (=I2xc3x97R, where I denotes the current and R denotes the internal resistance) while the heat related to chemical reactions during charging (i.e., exothermic reaction) and the heat related to chemical reactions during discharging (i.e., endothermic reaction) cancel each other. Therefore, the resistance of the electrolyte is reduced and a rapid increase of the voltage during charging is prevented. Therefore, a battery at a low temperature can be charged in a short time.
The present invention also provides a battery charging control method comprising the steps of:
executing a first pulsed charging and discharging operation when the temperature of a battery is lower than a first predetermined temperature (e.g., 0xc2x0 C. in the embodiment explained below), wherein in the first pulsed charging and discharging operation, the amount of charging is equal to the amount of discharging; and
switching from the first pulsed charging and discharging operation to a second pulsed charging and discharging operation when the temperature of the battery exceeds the first predetermined temperature, wherein in the second pulsed charging and discharging operation, the amount of discharging is less than the amount of charging.
According to this method, when the temperature of the battery is lower than the first predetermined temperature, the battery is heated due to a heating effect based on Joule heat. Therefore, the resistance of the electrolyte is reduced and a rapid increase of the voltage during charging is prevented. When the temperature of the battery exceeds the first predetermined temperature, the operation is switched from the first pulsed charging and discharging operation under the condition xe2x80x9cthe amount of charging=the amount of dischargingxe2x80x9d to the second pulsed charging and discharging operation under the condition xe2x80x9cthe amount of charging greater than the amount of dischargingxe2x80x9d; thus, accomplishing not only heating but also charging. Therefore, a battery at a low temperature can be charged in a shorter time.
Preferably, in the first pulsed charging and discharging operation, for lower temperatures of the battery, a set amount of charging in the charging interval and a set amount of discharging in the discharging interval are chosen to be smaller.
Accordingly, for lower temperatures of the battery (i.e., for larger resistances of the electrolyte), the amount of charging is set smaller, thereby very reliably preventing an increase of the voltage of a low-temperature battery being charged.
Typically, in the second pulsed charging and discharging operation, a set amount of discharging is determined by adjusting at least one of a pulse width and a pulse amplitude. Accordingly, the amount of pulsed discharging can be flexibly determined by suitably restricting the charging time or charging current of each pulse, and various user demands for charging control can be satisfied.
Also typically, the method further comprises the step of switching from the second pulsed charging and discharging operation to a normal charging operation of charging the battery at a continuous current when the temperature of the battery exceeds a second predetermined temperature (e.g., 10xc2x0 C. in the embodiment explained below).